Optical isolators are devices that allow radiation (usually light) to pass through them in one direction, but not in the opposite direction. In a typical optical-fiber-based system, a laser is coupled to an optical isolator through free space optics then through other free space optics to an optical fiber, through which the output of the laser is directed. Isolators are needed in optical systems in many roles, the most common of which is preventing reflections from returning toward, re-entering, and disrupting the operation of the laser. Optical isolators, then, are the optical analog of a diode in an electrical circuit.
The most basic optical isolators accomplish their task by rotating the state of polarization (i.e., its electric and magnetic field vectors) of the light entering the isolator. Most optical phenomena are reciprocal, however, so construction of an isolator is not necessarily straightforward. One optical phenomenon that is not reciprocal—and is therefore useful in constructing an isolator—is known as the “Faraday effect.” The Faraday effect occurs when certain materials, for instance Yttrium-Iron-Garnet (YIG), are placed in a strong magnetic field. Light traveling within the material has its state of polarization (i.e., its electric and magnetic field vectors) rotated by an amount depending on the length of the material and the strength of the magnetic field. The most important aspect is that the Faraday effect is nonreciprocal: light traveling in one direction may have its polarization rotated counterclockwise by, for example 45°, but light traveling in the opposite direction would have its polarization rotated clockwise by the same amount (45° in this example). Thus, the rotation is in opposite directions relative to the direction of the ray of light, but in the same direction in relation to the rotator. A device that uses the Faraday effect is commonly known as a “Faraday rotator.” A simple single-stage optical isolator consists of a Faraday rotator sandwiched between a pair of polarizing filters. Two-stage optical isolators are built by adding further rotators and polarizing filters to one-stage isolators.
For certain applications, it is desirable to have a polarization-maintaining isolator; in other words, an isolator in which the polarization state of the output is the same as the polarization state of the input. Two polarization states are considered the same if the orientation of the first state's polarization plane differs from the second state's polarization plane by 180° (π radians), or some integer multiple thereof. One approach commonly used in the prior art to build a polarization-maintaining isolator is to put a phase retardation plate (also known as a wave plate) following a two-stage optical isolator. In this type of isolator, one half the rotation of the state of polarization is accomplished by the two-stage isolator. The phase retardation plate accomplishes the final half of the overall rotation. Phase retardation plates, however, are designed for one specific wavelength and are very sensitive to variables such as manufacturing tolerances, meaning that the phase retardation plate introduces errors in the rotation of the polarization state, particularly in situations where the wavelength of the light is difficult to control, or where it is purposely varied. These errors can have a significant effect on the proper operation of existing isolators and limit their application.